Vaccination is one of the most effective methods for preventing infectious diseases. However, a single administration of an antigen is often not sufficient to confer full immunity and/or a long-lasting response. Approaches for establishing strong and lasting immunity to specific pathogens include addition of adjuvants to vaccines and/or repeated vaccination, i.e. boosting an immune response by administration of one or more further doses of antigen. Such further administrations may be performed with the same vaccine (homologous boosting) or with a different vaccine (heterologous boosting). The most common approach for homologous boosting is not only to administer the same vaccine, but also to administer it in the same dose as the earlier administration.
One disease for which multi-dose vaccination so far has been required is malaria. Malaria is one of the world's major health problems. For the year 2010, the World Health Organization reported an estimated 219 million cases of malaria globally. Malaria is caused by protozoan parasites of the genus Plasmodium. 
The life cycle of the parasite is complex, requiring two hosts, man and mosquito for completion. The infection of man is initiated by the inoculation of sporozoites through the saliva of an infected mosquito. The sporozoite stage has been identified as one potential target of a malaria vaccine. The major surface protein of the sporozoite is known as circumsporozoite protein (CS protein). RTS,S, an antigen based on the malarial CS protein and a viral envelope protein of the hepatitis B virus, has been under development for more than 25 years and is currently the most advanced malaria vaccine candidate being studied. Its structure and production were described in U.S. Pat. No. 5,928,902, issued 27 Jul. 1999.
In early work, RTS,S was tested in a small clinical trial in combination with an adjuvant comprising QS21 and 3D-MPL associated with an oil-in-water emulsion adjuvant (Stoute et al. 1997 NEJM 336:86). A three full-dose administration schedule had been planned for this study, but because of perceived excess reactogenicity, the third dose was reduced to ⅕ and administered later than originally planned. This study resulted in six out of seven subjects being protected. In subsequent work, a three full-dose immunisation schedule was used and, in more recent studies, also using a three full-dose immunisation schedule, RTS,S was adjuvanted with a liposomal formulation comprising QS21 and 3D-MPL. This adjuvant is termed AS01 and is described e.g. in WO 96/33739 and WO2007/068907. Recent data from a large-scale Phase III clinical trial, wherein RTS,S/AS01 was administered in three identical doses, one month apart, showed that over 18 months of follow-up, RTS,S/AS01 was shown to almost halve the number of malaria cases in young children (aged 5-17 months at first vaccination) and to reduce by around a quarter the malaria cases in infants (aged 6-12 weeks at first vaccination) over a follow-up period of 18 months.
While significant progress has been made in the field of vaccine research and development, there is still a need for novel immunogenic compositions and methods of immunising against diseases, including malaria, which are highly efficacious, safe, cost-effective, long-lasting and induce a broad spectrum of cross-reactive immune responses.